Reinduction (weeks 20-29): Patients < 13 years old receive dexamethasone 3 times daily on days 140-161. Patients ≥ 13 years old receive oral dexamethasone on days 140-146 and 155-161; nelarabine IV on days 169-173; vincristine IV over 15 minutes on days 140, 147, and 153; pegaspargase IM on day 143; cyclophosphamide IV on day 176; cytarabine IV or SC on days 176-179 and 183-186; oral thioguanine on days 176-189; and methotrexate IT on days 176 and 183. Patients with CNS disease undergo craniocervical radiotherapy beginning on day 196 and continuing for 7-10 days.